The Gulf Loop is the current that loops up, to the right of the middle of the Gulf of Mexico then drops down to the left of Florida where it then passes below Florida into the Atlantic, where it contributes to the Gulf Stream, which passes up the east coast of the United States and Canada.

Gulf loop stalled July 28, 2010

The concern now is whether natural processes can re-establish the stalled Loop Current

The Gulf Stream does not appear to be slowing down, say US scientists who have used satellites to monitor tell-tale changes in the height of the sea.

Confirming work by other scientists using different methodologies, they found dramatic short-term variability but no longer-term trend.

The research is published in the journal Geophysical Research Letters.

The stream is a key process in the climate of western Europe, bringing heat northwards from the tropics and keeping countries such as the UK 4-6C warmer than they would otherwise be.

It forms part of a larger movement of water, the Atlantic Meridional Overturning Circulation, which is itself one component of the global thermohaline system of currents.

Between 2002 and 2009, the team says, there was no trend discernible - just a lot of variability on short timescales.

The satellite record going back to 1993 did suggest a small increase in flow, although the researchers cannot be sure it is significant.

"The changes we're seeing in overturning strength are probably part of a natural cycle," said Josh Willis from Nasa's Jet Propulsion Laboratory (JPL) in California.

"The slight increase in overturning since 1993 coincides with a decades-long natural pattern of Atlantic heating and cooling."

The first observations suggesting the circulation was slowing down emerged in 2005, in research from the UK's National Oceanography Centre (NOC).

Using an array of detectors across the Atlantic and comparing its readings against historical records, scientists suggested the volume of cold water returning southwards could have fallen by as much as 30% in half a century - a significant decline.

The warm surface water sinks in the Arctic and flows back southwards at the bottom of the ocean, driving the circulation.

However, later observations by the same team showed that the strength of the flow varied hugely on short timescales - from one season to the next, or even shorter.

But they have not found any clear trend since 2004.

Argo float being deployed

The NOC team now has a chain of instruments in place across the Atlantic, making measurements continuously.

"In four-and-a-half years of measurement, we have found there is a lot of variability, and we're working to explain it," said NOC's Harry Bryden.

The quantities of water involved are huge, varying between four million and 35 million tonnes of water per second.

The array is part of the UK-funded Rapid project, which aims to refine understanding of potentially large climate change impacts that could happen in short periods.

Professor Bryden's team calculates that their system is good enough to detect a long-term change in flow of about 20% - but it has not happened yet.

He believes the JPL approach - using satellite altimeters, instruments that can measure sea height precisely, and the Argo array of autonomous floating probes - could potentially add useful data to that coming from long-term on-site monitoring arrays.

But, he points out: "The method concentrates only on the upper [northward] flow - it doesn't give you much information on the returning flow southward."

What is the thermohaline circulation?

There are three main processes that make the oceans circulate: tidal forces, wind stress, and density differences.

The density of sea water is controlled by its temperature (thermo) and its salinity (haline), and
the circulation driven by density differences is thus called the thermohaline circulation.

The animation and list below describes the key features of the global-scale thermohaline circulation.

The Gulf Stream (and its extension, the North Atlantic Drift)
bring warm, salty water to the NE Atlantic, warming western Europe.

The water cools, mixes with cold water coming from the Arctic Ocean,
and becomes so dense that it sinks, both to the south and east of Greenland.

If we zoom out, we see that this current is part of a larger system,
connecting the North Atlantic...

...the tropical Atlantic...

...the South Atlantic...

...the Indian and Pacific Oceans...

...and the Southern Ocean. Further sinking of dense water occurs near to
Antarctica.

Click on image if gif does not show 11 sequences  click image to repeat

If we look below the surface, water from the two main sinking regions
spreads out in the subsurface ocean...

...affecting almost all the world's oceans at depths from 1000m and below...

The cold, dense water gradually warms and returns to the surface,
througout the world's oceans.

The surface and subsurface currents, the sinking regions, and the return of
water to the surface form a closed loop, the thermohaline circulation
or global thermohaline conveyor belt

Will the thermohaline circulation collapse?

The critical part of the thermohaline circulation (THC) is the sinking in the North Atlantic Ocean.

This occurs here (and not in the North Pacific) because the Atlantic is much more saline (and hence, denser).

It is more saline because it is warmer (more evaporation of fresh water increases the salinity of the sea water).

It is warmer in the North Atlantic because warm water is brought by the thermohaline circulation from the tropical and South Atlantic.

To some extent, therefore, the THC appears to be self-sustaining.

And if some event occurs to break this self-sustaining chain of processes, then there is the potential for the circulation to break down rapidly (i.e., over several decades) and to remain in a reduced-circulation state for several centuries.

Models from past

Some fairly simple models of the world's oceans do simulate a rapid break down of the THC, when the density of the water in the North Atlantic Ocean is lowered by adding fresh water (rain) and/or by warming.

Increased rainfall and warming over the North Atlantic are both expected as a result of increased greenhouse gas concentrations, and so it can be argued that global warming may cause a rapid collapse of the thermohaline circulation.

The self-sustaining system described above is, however, much more complex in reality, and the more complete climate models, that take some of these complexities into account, generally simulate only a gradual weakening of the THC in response to global warming.

Nevertheless, observations and palaeoclimate evidence both indicate that the THC has fluctuated both recently and in the
distant past.

When?

The majority of climate scientist believe that a critical change in the THC is unlikely to occur during this century, but the question cannot be answered with certainty at present.

Due to the potentially serious impact on our climate of a collapse of the THC, it must be regarded as a low-risk,
high-impact event that cannot be ignored.

What would happen to our climate if the thermohaline circulation collapsed?

Due to the interactions between many components of the climate system, it is not a simple matter to estimate how different our climate would be without the current thermohaline circulation.

Certainly the biggest impact would be on the temperature over the North Atlantic and Europe.

The northern North Atlantic and NW Europe have annual temperatures that are about 9 degrees C above the average for their latitude, but this cannot all be attributed to the THC, since the wind driven ocean circulation helps to transport heat
to these regions too.

This represents an upper bound on the cooling that would occur following a collapse of the THC.

Simulations with a complex climate model (see figure below, courtesy of Michael Vellinga), whose thermohaline circulation is forced into a collapse, show cooling of around 8 degrees C around the coast of Greenland, but with more moderate cooling elsewhere (less than 2 degrees C over most of Europe).

What must be remembered is that these changes will be superimposed on the pattern of warming due to the enhanced greenhouse effect.

Over Europe, then, little change or slight warming would probably be expected if the THC collapsed, with cooling restricted to the ocean areas.

Associated changes in storminess and precipitation are even harder to estimate.

Change in annual temperature 30 years after a collapse of the thermohaline
circulation

A resident walks his dogs past ice sculptures near the Ice Hotel in Jukkasjarvi, above the Arctic Circle in northern Sweden, December 24, 2006.

The US government has listed polar bears as a threatened species owing to a drastic reduction in Arctic sea ice, but insisted the step did not mark a policy shift to attack global warming.

Photo: REUTERS/Bob Strong

Frozen Torne RiverJukkasjarvi

Thermohaline circulation Gulf Stream shutdown

A dogsled team carries tourists down the frozen Torne River in Jukkasjarvi, above the Arctic Circle in northern Sweden, December 24, 2006.

Photo: REUTERS/Bob Strong

Thursday, 24 May 2007

Mission to Ice Island

By David Shukman

BBC science correspondent, Canadian Arctic

Our science correspondent David Shukman has been investigating a new geographical feature in the Far North  an island of ice, seen by many scientists as one of the most dramatic signs of warming in the Arctic. Follow his journey on this page.

Heading home: the BBC trio prepare to turn south

DAY SIX  HEADING SOUTH

We reach the Arctic staging-post of Resolute on the first stage of our journey home, all a bit stunned by a combination of adrenalin, exhaustion, sunburn and cold.

After landing on the Ice Island, we'd then had to work through the night editing our reports for the next day's broadcasts so now we all keep dozing off at every turn.

We visit Qarmartalik School here in Resolute, the hallway filled with snowboots, the corridors and schoolrooms brightly lit and decorated.

The two scientists we're with, Derek Mueller and Luke Copland, give a presentation to a class of teenagers, mostly Inuit, to explain what we've been doing.

Up on a projector screen appear images of the Arctic  the retreat of the sea ice, the temperature graphs showing the extraordinary recent warming, the break-up of the Ayles Ice Shelf now adrift as an island.

This is their land that is changing

The students seem to be spellbound  this is their land that's changing.

They ask sharp questions.

How long will it take for the Ice Island to melt? Answer  it could be years.

Where will it go?

Probably west to Alaska but it could break into chunks, one of which could even end up in the bay right here.

The principal, Brian Manning, tells me how he always encourages passing scientists and explorers to visit the school, to open the children's eyes to the world outside.

An Arctic wolf is a magical sight

I ask one of the teachers to write out the words "Ice Island" in Inuktitut, the Inuit language. She asks, which kind of ice?

Thin sea ice or glacial ice?

Just as there are many ways to describe snow, so there are huge differences in types of ice.

The island is actually made up of a mixture of glacial ice and old sea ice that's accumulated over many years.

The teacher consults a few others and, for the record, she settles on "aujuittuq qikiqtaq".

As we get ready for the next series of flights, we wonder at the huge interest this story has provoked.

Not all of it favourable.

We've had some very challenging questions about how we justify so much flying.

Luke Copland argues that there's a limit to what satellite analysis can tell you  only by landing on the island was it possible to measure its depth and to position the tracking beacon.

Derek Mueller agrees, and also says he has already calculated the exact mileage and offset the carbon cost with a green project in Canada.

For myself, I'll also offset the carbon cost.

But more than that, I hope we've provided a service by reporting on the first scientific research mission into the largest ice break-up in the Arctic for 40 years.

Meanwhile odd memories of the last extraordinary few days keep flashing to mind.

From the air, on our return from the island, a long thin winding trail across the icy white below had turned out to be the path of a polar bear on the hunt for seals.

At the Eureka weather station, our home for nearly a week, a short walk to stretch our legs had brought us to within a snowball's distance of a pack of Arctic wolves.

First one came by, padding over the ice, sniffing our smell and moving on; then the others, the braver ones edging a bit closer, a magical sight, but never too close.

And then the Ice Island itself  so vast, so dazzling, visited briefly and noisily, and now sliding silently across the ocean, enormous enough to become a new feature on the Arctic map but ultimately destined to vanish.

After days of waiting and fretting over satellite pictures showing confusing swirls of clouds, we take the plunge and set off for our second attempt to land on the Ice Island.

This time the clear skies over our base here at Eureka continue over the mountains and fjords and  crucially  over the sea.

I stand in the cockpit door and watch amazed as a sheet of white appears before us without a single cloud obscuring the view.

At first, it's hard to tell exactly where the Ice Island is  there's such a jumble of cracks and icebergs.

But then, one vast area of ice appears far smoother than the rest and this is confirmed as the ice island itself.

Excitement mounts as we circle at ever lower altitudes.

The pilot, Rodney Fishbrook, is searching for landing sites.

He turns in his seat and signals that we will touch down.

We buckle up very firmly  we've been warned about a bumpy landing.

When it comes, the moment is far more dramatic than I'd expected  a roar of the engines, a storm of ice around the windows, a series of violent lurches as the skis ride over the mounds of snow.

The Ayles Ice Island calved off the Ayles Ice Shelf in August 2005

The calving event was the largest in at least the last 25 years

A total of 87.1 sq km (33.6 sq miles) of ice was lost in this event

The largest piece was 66.4 sq km (25.6 sq miles) in area

This made the slab a little larger than Manhattan

Ayles Ice Island has moved some 50km (30 miles) since calving

Suddenly we're silent, stunned. Producer Mark Georgiou is the first to remember his manners and leads us in a round of applause for Rodney.

We climb out into a world of brilliant sunshine and endless white. None of us can cope without sunglasses  the glare is so intense.

The island stretches for miles around us.

The air is amazingly still  which is lucky because when we first land the temperature is minus 18.

Some of the surface is firm to walk on, other parts are deep with snowdrifts.

Jumbles of huge chunks of ice mark the areas where cracks have opened and reformed.

There are curious reminders of where the island came from.

Luke digs down and extracts a tiny lump of algae  a lifeform that grew up on what was an ice shelf and which now finds itself a passenger on a drifting island.

Derek breaks off shards of ice from under the snow and reminds me that they're 3,000 years old.

I find one thought particularly evocative: that I'm visiting an incredibly beautiful and remote part of the world which will soon vanish.

The ice that's floating beneath me will eventually melt.

And another corner of the warming Arctic will be gone forever.

Recent satellite images show the island free of the coast

For Eureka scientists, the BOAC women were tantalisingly close

DAY FOUR  WAITING AT EUREKA

A day of endless white and endless waiting.

A huge depression stretching from here to the North Pole is swirling its way over us and the weather maps position the word "Low" exactly over the current location of the Ice Island we're trying to land on.

We cheer ourselves with talk of how it's bound to clear soon.

And with time on our hands we hear the tales of this lonely base, and we all enjoy this one: the tale about Eureka and stewardesses with BOAC, the forerunner of British Airways.

It's a story of longing and glamour from a more innocent age.

It turns out that in the 1960s, the polar aviation routes took many passenger planes directly over Eureka.

As the jets passed overhead, the aircrews were obliged to check in and report their positions.

Well, during the long dark winters, the weathermen down in the ice below, sheltering in the storm-battered buildings here, had a bright idea: to run a competition to find the "personality queen" among the stewardesses flying above them.

Sue Curtis' letter won the hearts of her "deep freeze darlings"

Each time the radios crackled into life, the weathermen would crowd around and try to strike up a conversation.

It can't have been easy, with the flight crew and the entire base listening.

But this remote dialogue through the icy air was clearly popular.

I have before me an adored album of cheery letters and photographs of pretty women in miniskirts sent by the BOAC stewardesses to the Eureka base.

Wendy Madley apologises for the fact that none of them chose to send pictures of themselves in swimming costumes.

"Still," she writes, "that might prove too much for folk who are virtually womanless for such a long time!"

Margaret van der Linde asks if any of the weathermen enjoy her favourite song of the year  1969  which is "Sugar, Sugar", though she's not sure who it's by.

But the winning line comes from Sue Curtis, who eventually won the competition.

She begins her letter with the greeting: "Hello my deep freeze darlings!"

Ron Girardin was sent to London to present Sue Curtis with an award

She also includes some rather coy "statistics" about her height and weight, which almost certainly weren't the figures the men in the Arctic were supposed to be interested in.

Anyway, Sue Curtis stole the hearts of the Eureka staff and one of them, a technician, Ron Girardin, then aged 25, made the long journey to London to hand over a trophy to her.

The good news is that we've flown out over the Arctic Ocean and have actually seen the ice island.

The bad news is that the cloud was too low for the pilot to attempt a landing on it.

We took off from Eureka in glorious evening sunshine and crossed a spectacular landscape of frozen valleys and mountaintops poking out through blankets of mist.

A fjord was bristling with craggy icebergs, locked in place until the summer thaw in a couple of months' time.

But as we approached the coast, we saw the first clouds.

And soon everything was obscured by a dense floor of fluffy white.

It was a depressing moment.

Just before taking off we had consulted the latest satellite images.

They had shown that the cloud was clearing to the east so in theory the island should have been clear too by the time we got there.

Instead we found ourselves descending through the cloud into a world of white, with the white ice of the sea blurring with the white of the sky.

It turns out that the weather system had not kept moving as predicted  it had stalled exactly where we didn't want it.

A spectacular landscape was illuminated by the evening sun

And with no horizon as a guide, there would be no touchdown.

Heavy hearts all round.

With visibility so bad, it took a while to find the island.

We banked and turned and frantically checked the maps and satellite pictures.

And then an edge of it came into view, a ragged dark line separating the 3000-year-old ice of the island from the fresher ice of the frozen sea.

And then another telltale sign that we were in the right place: a line of boulders, big lumps of black, hitchhikers from the ice island's original position attached to the coast, now journeying who knows where.

We made another couple of passes, the pilot, Rodney Fishbrook, taking us down to 500ft (152m).

But with the light unchanging, he had no clear sense of the view ahead and we had no choice but to head back to Eureka, the scientists' drills untouched, the satellite tracking beacon still packed in its case, strapped to the floor of the plane.

Poor visibility prevented a landing

Even the sandwiches readied for a long night on the ice were untouched.

The Arctic is never easy, the veterans keep telling us.

No one here was too surprised that we hadn't made it first time.

Now the waiting begins again  and the obsessive scanning of the weather forecasts.

Another weather front is approaching so there'll be no chance for the next 24 hours.

Today's journey takes us across unremitting miles of snow, ice and mountain, from one remote spot to an even remoter one.

We travel by that stalwart of polar aviation: the Twin Otter.

We had hoped to film what's billed as a spectacular landscape (the northern regions of Ellesmere Island), but cloud gets in the way and the throb of the engines soon sends me into the deep sleep that I didn't get last night.

The eyeshades didn't do enough to shade the midnight sun, which blazed through the curtains.

I feel a bit guilty dozing over terrain that proper explorers tackled on foot.

I've read the unnerving tales of dog teams vanishing in blizzards and ships getting crushed in the ice.

But then we land at the Eureka High Arctic Weather Station and all I feel is relief.

This is the world's northernmost, permanently manned civilian outpost, perched on the last stretch of land before the North Pole  and it's an oasis of civilisation.

A sign says that although we've made it more than 6,000 miles (9,600km) from London, we're still 1,000 miles (1,600km) from the North Pole.

Even so, it's far enough North for the Sun never to set.

It just seems to roam around the horizon and prompts people to check that we've brought the eyeshades essential to getting any sleep here.

Another source of questioning is why on Earth we have brought luggage emblazoned with the markings of the British Antarctic Survey (BAS).

Wrong end of the planet surely?

Well, the answer is that at their Cambridge headquarters the BAS agreed to kit us out for our journey  so I'm sure we'll be warm enough but I know that explaining ourselves to everyone we meet will become a bore.

Two scientists travelling with us, Luke Copland of the University of Ottawa and Derek Mueller of the University of Alaska Fairbanks, know the score already.

We've come a long way north  but we've still got a long way to go

They've also agreed to our ban of another topic of conversation: the weather.

Everything about this trip hinges on clear skies and we realised early on that we could talk ourselves deep into the snowdrifts speculating about whether a break will appear when we need it.

First, we have to fly from here to the Eureka station another few hundred miles north.

And then from there we need a seemingly improbable combination of weather conditions to be able to reach our destination: the Ayles Ice Island, a monster iceberg 10 miles long and three miles across (16km by 5km), that tore away from the Arctic coastline in one of the most dramatic events of recent times, altering the very geography of this region.

Although the break-up happened in August 2005, news only emerged from scientific circles a few months ago and  until now  no scientists have had the chance to investigate this weird new structure, let alone any journalists.

Computer models of climate have regularly predicted that the north Atlantic conveyor may well reduce in intensity or even turn off altogether, a concept that was pushed beyond credence in the Hollywood blockbuster The Day After Tomorrow.

What happens is that as Arctic ice melts and Arctic rivers flow faster  trends which have both been documented  the northern oceans become less saline.

Less salinity means a lower density; the waters then cannot sink, so the conveyor weakens.

Computer models have predicted that if it turned off completely, Europe would cool by perhaps four to six degrees Celsius.

Commenting in Nature, Detlef Quadfasel from the University of Hamburg writes that the NOC experiments provide "...the first observational evidence that such a decrease of the oceanic overturning circulation is well underway."

Natural variation

The NOC researchers admit that the case is not yet proven.

The analysis involves only five sets of measurements, made in 1957, 1981, 1992 and 1998 from ships, and in 2004 from a line of research buoys tethered to the ocean floor.

Even if the trend is confirmed by further data, it could be down to natural variability rather than human-induced global temperature change.

"This issue of variability is very important," said Harry Bryden, "and we do not have any good grasp of it.

"Models can predict it, but we think we ought to go out and measure it."

Michael Schlesinger from the University of Illinois at Urbana-Champaign, a leading expert in models of climate and ocean circulation, believes that even with these caveats, the NOC team has probably come up with a link to human-induced climate change.

"The variability question is the right one to ask," he told the BBC News website, "but the phasing is wrong."

A decade ago Professor Schlesinger showed that the north Atlantic conveyor undergoes a natural 70-year cycle of strengthening and weakening.

"The Bryden measurements are out of phase with this cycle," he said.

In 2004 buoys were deployed from ships onto tethers

"The natural cycle had a northern cooling until the mid-1970s and a warming afterwards, and here we see an apparent cooling."

He is also convinced by other details of the NOC measurements showing that the changes in the southerly underwater flow have occurred at great depths.

"The slowing down of the southward return occurs between 3,000 and 5,000m; and this more or less constitutes a smoking gun," he said.

Choosing policies

So what does all this mean for European weather? Will it necessarily get colder  or will the apparent recent trend of warmer summers continue?

"If this trend persists," said Harry Bryden, "we will see a temperature change in northern latitudes, perhaps of a degree Celsius over a couple of decades."

But climate is a complex phenomenon; other factors could conspire, even so, to produce a net warming.

Models can predict variability, but we think we ought to go out and measure it

Harry Bryden

"The UK government is looking, in terms of mitigating climate change and adapting to it, at a warming scenario," said Phil Newton of the UK's Natural Environment Research Council, which funds the Rapid investigators.

"You might now be asking what sort of mitigation and adaptation they should be looking for."

To answer this question, the Rapid team plans to continue their measurements in the next few years.

Their buoys remain in place, and ships can go to gather their data as often as finance allows.

The findings will have resonance beyond the shores of the UK and Europe, as extra heat left circulating around the tropical Atlantic could have major impacts on weather systems in Africa, the Caribbean and central America.

Walking on ice

Thermohaline circulation Gulf Stream shutdown

A polar bear mother and her two cubs walk along the shore of Hudson Bay in Manitoba near Churchill, Canada.

The US government has listed polar bears as a threatened species owing to a drastic reduction in Arctic sea ice, but insisted the step did not mark a policy shift to attack global warming.

Photo: THE CANADIAN PRESS/Jonathan Hayward

Hudson BayChurchill, Canada

Thermohaline circulation Gulf Stream shutdown

A polar bear sits on the Hudson Bay in 2007 outside Churchill, Canada.